In semiconductor production, wafers are sequentially processed in a plurality of process steps during the production process. With increasing integration density, demands in terms of the quality of the features configured on the wafers are rising. It is advantageous, for this purpose, if the quality of even individual process steps, for example lithography steps, can be reliably assessed during the manufacturing process and before any subsequent process step. Thus, if a determination is made, just after a process step is performed and even before a production process has been completed, that a wafer or features configured on the wafer are defective, the wafer can be immediately discarded with no need to perform additional subsequent process steps. Or the wafers found to be defective can be reprocessed separately until satisfactory quality is achieved. Efficiency and yield in semiconductor production can thereby be enhanced.
Optical apparatuses are particularly suitable for inspecting the surface of wafers. Optical apparatuses are known that, by image recognition, can recognize a wide variety of features on the surface of a wafer. Wafer inspection apparatuses of this kind operate either in a bright-field configuration, in which the surface of the wafer is illuminated and the light reflected from the surface is sensed by a camera, or in a dark-field configuration, in which the surface of the wafer is illuminated and the light scattered from defects, particles and the like is sensed by a camera. In a wafer inspection apparatus of this kind, the surface of the wafer can also be illuminated stroboscopically, i.e. with short flashes of light.
Images acquired in a bright-field and a dark-field configuration are informative in different ways. For optimum wafer inspection, it is therefore desirable to sense images with a wafer inspection apparatus in both a bright-field and a dark-field configuration. In order to achieve high accuracy in the detection of defects on the surface of wafers in a dark-field configuration, a high intensity of the light flashes used to illuminate the surface is desirable.